This invention relates generally to natural gas processing and to mixed gas refrigeration systems.
Natural gas transmission pipelines are typically operated under a very high pressure, which can range between 200 to 1000 pounds per square inch gauge (psig). At various locations all over the pipeline network, known as let-down stations, this high pressure gas is throttled down to a lower pressure which is more suitable for its end-use. This low pressure will typically range between 40 to 80 psig. The throttling action of the gas can actually reduce the gas temperature to below 32xc2x0 F. and hence pipe-freezing and frost formation is a problem that has to be avoided. A standard solution takes a small fraction of the natural gas and burns it to produce hot gas which is then directed on to the pipe surface to prevent freezing. As a result, the free pressure energy available from letting down the gas pressure is typically not utilized in any useful form.
Accordingly it is an object of this invention to provide a method for gainfully employing pressure energy found in natural gas processing systems.
The above and other objects, which will become apparent to those skilled in the art upon a reading of this disclosure, are attained by the present invention, one aspect of which is:
A method for producing liquefied natural gas comprising:
(A) cooling a first natural gas stream to produce first cooled natural gas, and expanding the first cooled natural gas to produce refrigeration bearing natural gas;
(B) cooling a second natural gas stream by indirect heat exchange with the refrigeration bearing natural gas to produce second cooled natural gas;
(C) compressing a mixed refrigerant fluid, cooling the compressed mixed refrigerant fluid, and expanding the cooled mixed refrigerant fluid to produce refrigeration bearing mixed refrigerant fluid;
(D) warming the refrigeration bearing mixed refrigerant fluid by indirect heat exchange with the cooling compressed mixed refrigerant fluid and by indirect heat exchange with second cooled natural gas to condense at least some of the second cooled natural gas; and
(E) recovering resulting condensed natural gas as product liquefied natural gas.
Another aspect of the invention is:
A method for producing liquefied industrial gas comprising:
(A) cooling a natural gas stream to produce cooled natural gas, and expanding the cooled natural gas to produce refrigeration bearing natural gas;
(B) cooling an industrial gas stream by indirect heat exchange with the refrigeration bearing natural gas to produce cooled industrial gas;
(C) compressing a mixed refrigerant fluid, cooling the compressed mixed refrigerant fluid, and expanding the cooled mixed refrigerant fluid to produce refrigeration bearing mixed refrigerant fluid;
(D) warming the refrigeration bearing mixed refrigerant fluid by indirect heat exchange with the cooling compressed mixed refrigerant fluid and by indirect heat exchange with cooled industrial gas to condense at least some of the cooled industrial gas; and
(E) recovering resulting condensed industrial gas as product liquefied industrial gas.
As used herein the term xe2x80x9csubcoolingxe2x80x9d means cooling a liquid to be at a temperature lower than the saturation temperature of that liquid for the existing pressure.
As used herein the term xe2x80x9cindirect heat exchangexe2x80x9d means the bringing of fluids into heat exchange relation without any physical contact or intermixing of the fluids with each other.
As used herein the terms xe2x80x9cturboexpansionxe2x80x9d and xe2x80x9cturboexpanderxe2x80x9d means respectively method and apparatus for the flow of high pressure fluid through a turbine to reduce the pressure and the temperature of the fluid thereby generating refrigeration.
As used herein the term xe2x80x9cvariable load refrigerantxe2x80x9d means a mixture of two or more components in proportions such that the liquid phase of those components undergoes a continuous and increasing temperature change between the bubble point and the dew point of the mixture. The bubble point of the mixture is the temperature, at a given pressure, wherein the mixture is all in the liquid phase but addition of heat will initiate formation of a vapor phase in equilibrium with the liquid phase. The dew point of the mixture is the temperature, at a given pressure, wherein the mixture is all in the vapor phase but extraction of heat will initiate formation of a liquid phase in equilibrium with the vapor phase. Hence, the temperature region between the bubble point and the dew point of the mixture is the region wherein both liquid and vapor phases coexist in equilibrium. In the practice of this invention the temperature differences between the bubble point and the dew point for the variable load refrigerant is at least 10xc2x0 K, preferably at least 20xc2x0 K and most preferably at least 50xc2x0 K.
As used herein the term xe2x80x9cindustrial gasxe2x80x9d means a fluid having a normal boiling point of 150xc2x0 K or less. Examples of industrial gases include nitrogen, oxygen, argon, hydrogen, helium, neon and fluid mixtures containing one or more thereof.
As used herein the term xe2x80x9cnatural gasxe2x80x9d means a fluid comprised of at least 45 mole percent methane.